Scientists Say: Pyrolysis

Pyrolysis (noun, “py-RAW-leh-sis”)

Pyrolysis is a process though which heat breaks down large molecules into smaller ones. It is just one of the chemical reactions that occurs when something is burning.

Picture a campfire. The flame may look like a single chemical reaction. But in reality, it comes from multiple reactions that occur step by step.

The first step is pyrolysis. In this step, heat breaks open the fuel’s chemical bonds. In the case of a campfire, that fuel is wood. But pyrolysis doesn’t create the campfire’s flame. The next step — combustion — does that. Combustion occurs when the broken-open fuel molecules react with oxygen in the air.

Pyrolysis can occur without combustion. For example, if you seal wood in an oxygen-free container and heat it, the wood won’t ignite. Instead, pyrolysis will slowly transform it into charcoal. Pyrolysis typically does not break down fuel as completely as combustion does. That’s why charcoal can still burn to heat up a grill. In contrast, soot collected from a cold campfire typically cannot be burned.

Pyrolysis also plays a crucial role in cooking up a popular sweet treat: caramel.

To make caramel, a chef heats sugar and water on a stovetop into a syrup. As the syrup gets hotter and hotter, the sugar molecules begin to break open. This causes the clear syrup to turn yellow-gold and then a deep, caramel brown. That’s the chef’s cue to take the caramel off the heat. They may then add butter and cream to the hot, gurgling caramel. Much of the chemical transformation that turned syrup into caramel occurred through heat alone. Therefore, it is an example of pyrolysis.

In a sentence

Pyrolysis can produce biochar, a valuable, carbon-rich substance that farmers use to improve soil quality.

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